Polyamine-epichlorohydrin resins and polyamide amineepichlorohydrin resins having a reduced organic chlorine content are industrially available, the application or product properties, in particular the effectiveness of the products, being retained virtually in full. Such resins have long been employed for improving the dry and wet strength of paper. Cationic polycondensates containing hydrophobic radicals, for example based on fatty acid-modified polyamines, are also suitable as sizing agents for paper. The organic chlorine content is composed of a certain amount of chlorohydrin functions bonded in the resin, and a chlorine component originating from the low-molecular-weight secondary components chloropropanediol and dichloropropanediol.
When used as a wet-strength agent, an aqueous solution of the polycondensate is added to a pulp suspension, from which the sheet of paper is formed by dewatering. If the resins are adsorbed incompletely onto the cellulose, in the case of a halogen-containing wet-strength agent a certain proportion of organic halogen compounds pass into the effluent from the papermaking, and can be detected there as the so-called AOX value (AOX=adsorbable organic halogen) (DIN 38 409 H14). For environmental protection reasons, introduction of AOX into the waste-water from paper-mills should be avoided or at least kept as low as possible.
Although the content of the secondary component in many products is already well below 0.1%, it is desired to employ chlorine-free products.
Halogen-free papers cannot be produced using the above-mentioned auxiliaries. The term "halogen-free" here should be used in the strictest sense only for products which really do contain no halogen.
Methods are furthermore already known from the prior art for chlorine-free provision of a wet-strength finish, isocyanates being employed as the raw materials. The following procedures are possible here: 1. treatment of paper with masked polyisocyanates, 2. use of reactive mixtures which react in the substrate to give polyurethane, 3. treatment of paper with free polyisocyanates in organic solvents.
However, the known wet-strength agents are not satisfactory in all requirements.
DE-A 3 102 038 describes basic polyurethanes which contain isocyanates which are blocked via oxime groups and are split off during reaction with the cellulose. The disadvantage of this method is that organic radicals, which are not substantive with respect to cellulose, may enter the circulating water of the paper machinery.
A two-component system for coating paper, comprising a polyfunctional isocyanate and a compound which is reactive to isocyanates and contains active hydrogen, is proposed in FR-A 2 360 714. At the end, the finished paper contains 0.5 to 35% by weight of reacted polyurethane.
EP-A 0 017 598 describes aqueous dispersions of polyurethanes, the isocyanate groups of which are blocked. After application to the substrate, the isocyanate groups must be demasked by addition of a metal catalyst or a salt of a tertiary amine. The isocyanate groups thereby released react with the cellulose during the heat treatment.
U.S. Pat. No. 3,702,781 describes polyurethanes having reactive NCO groups, which are applied to paper in organic solvents.
In the process according to DE-A 3 102 038, the isocyanate group must be produced in situ by deblocking protective groups. A disadvantage here is that the protective groups have no affinity for the fibre and therefore can be introduced into the water circulation of the paper machinery. Moreover, higher temperatures (&gt;130.degree. C.) than those which the paper reaches while passing through the drying part of the paper machinery are required for the deblocking. The contact times in the drying part (&lt;60 seconds) as a rule are not sufficient to achieve quantitative deblocking. In the case of U.S. Pat. No. 3,702,781, in which free isocyanates are used, it is necessary because of the high viscosity to use organic solvents, which have to be disposed of by the paper producer and therefore should be avoided as far as possible.
Water-dispersible polyisocyanate formulations which contain free NCO groups and are suitable as an additive for aqueous adhesives are known from EP-A 0 206 059.
DE-A2 557 409 discloses resin acid isocyanates which can be dispersed in water in the presence of nonionic and anionic emulsifiers.
The dispersions or emulsions of carbamoyl sulphonates made from isocyanates and alkali metal and ammonium bisulphites and proposed in DE-A2 839 310can be employed as sizing agents.
U.S. Pat. No. 4,505,778 discloses bulk and surface sizing agents which comprise mixtures of aromatic polyisocyanates containing 1-10% by weight of an isocyanate prepolymer obtained, for example, by reacting aromatic polyisocyanates with monofunctional polyethers.
Furthermore, various polyurethane or polyurea dispersions are known which can be employed as sizing agents. However, these usually anionic products (cf. German Offenlegungsschrift 2 457 972) do not contain any reactive groups in the form of isocyanate groups. Examples of such products are given in the following documents: FR-A1 496 584, U.S. Pat. No. 3,989,659, DE-A2 537 653, EP-A0 037 379, DE-A3 438 563 and EP-A0 207 414.
Latices based on diurethanes have also been proposed as sizing agents (Of. EP-A0 232 196).
However, these products are not satisfactory in all respects. In particular, the stability or dispersibility in water is often poor or cationic additives are necessary to improve the inherent retention of the resin. EP-A0 074 544 describes dispersions which contain a disperse phase of 1. reinforced resin size and 2. hydrophobic ketene dimer or hydrophobic isocyanates having at least 12 carbon atoms, where a cationic dispersant (inter alia a polyamide amine-epichlorohydrin resin) is employed to improve the properties.
In addition, it would be advantageous for the user to have an auxiliary which provides the paper with wet and dry strength and simultaneously also with partial hydrophobicity, i.e. is suitable as a bulk or surface sizing agent.
There was therefore the object of providing a new process for providing cellulose-containing material with a wet- and dry-strength finish and/or sizing by means of a chlorine-free product which is based on unblocked polyisocyanates, can be emulsified in water without addition of emulsifiers and achieves the wet-strength action of the polyamidoamine/epichlorohydrin resins when used in a wide pH range in pulp and on the surface.
It has now been found, surprisingly, that water-dispersible polyisocyanate mixtures which contain tertiary amino groups, which can also be in the form of their salts, i.e. in protonated or quaternised form, and optionally contain hydrophobic groups and optionally polyether units, are highly suitable dry- and wet-strength agents for cellulose-containing material and in addition are also suitable as sizing agents i.e. for partial hydrophobisisation of cellulose-containing material, it being possible for them to be used before sheet formation (use in the pulp), that is to say as an additive to the fibre suspension, or on the surface, that is to say as an application to a sheet of paper which has already been formed.